The optimum dimensions of a laser discharge tube depend on the particular laser amplifying medium employed, but each type is limited to a maximum size beyond which efficient operation is not attainable.
The laser power obtainable from a gas discharge laser of the copper vapour type having a given length of discharge type can be increased whilst maintaining good efficiency by merely increasing the diameter of the discharge tube, because the power obtainable is proportional to the volume of the laser amplifying medium. However, at diameters greater than about 6 to 8 cm, the further power obtainable increases by only a small amount and the efficiency at which it is generated decreases as the diameter increases. In other words, for diameters greater than 6 to 8 cm or so, the laser power obtainable ceases to be proportional to the increase in volume of the laser amplifying medium. One of the principal reasons for this is that the lower laser level is metastable and is, therefore, depopulated by collisional rather than radiative processes. It is essential for efficient operation that collisional de-excitation of the atoms in the metastable state of the lower laser level is completed in the period between the pulses which excite the laser so that the maximum population inversion attainable is achieved by each pulse. Any lingering metastables (atoms in the lower laser level) will reduce the population inversion achieved and thus will reduce the laser efficiency. If the diameter of the discharge tube is too large, the volume to surface ratio becomes too great for effective de-ionisation and depletion of lower laser levels to occur by collision between excited atoms or molecules and the walls of the tube.